Although you add the int values as primitive types, rather than Integer objects, to li, the code compiles.
Because li is a list of Integer objects, not a list of int values,
you may wonder why the Java compiler does not issue a compile-time error.
The compiler does not generate an error because it creates an Integer object from i and adds the object to li. Thus, the compiler converts the previous code to the following at runtime:

Converting a primitive value (an int, for example) into an object of the corresponding wrapper class (Integer) is called autoboxing. The Java compiler applies autoboxing when a primitive value is:

Passed as a parameter to a method that expects an object of the corresponding wrapper class.
Assigned to a variable of the corresponding wrapper class.
Consider the following method:

public static int sumEven(List<Integer> li) {
int sum = 0;
for (Integer i: li)
if (i % 2 == 0)
sum += i;
return sum;
}
Because the remainder (%) and unary plus (+=) operators do not apply to Integer objects, you may wonder why the Java compiler compiles the method without issuing any errors. The compiler does not generate an error because it invokes the intValue method to convert an Integer to an int at runtime:

absolute value of -8 = 8
pi = 3.1416
Autoboxing and unboxing lets developers write cleaner code, making it easier to read. The following table lists the primitive types and their corresponding wrapper classes, which are used by the Java compiler for autoboxing and unboxing: